Automatic frequency control



Feb. 11, 1941. G. MOUNTJOY AUTOMATIC FREQUENCY CONTROL Filed April 15,'1938 w m m MK I l .H u. R T ..1. u IW l H ww 4m N 0 W um S1 HJQ nw R wm!N s Q) mm WMM. Wsw @www w+ QN) S .i ...T NwQQlzN r F NN QN .w m m A Mim.. bl\ HS-nun r Ihn .enbl m W m if hmmm w w J 25+ N IUHII m H u u HES.f u w M M n w A. W s MET" W W Jl@ (Q3 :S m m E9@ "ml f f www -r v sv .lNm. WL. m \m.u nu f m NN (E wm N A LII fz 1 /Lxi Patented Feb. 11, 1941UETED STATES PATENT 5.11,

Garrard Mountjoy, Manhasset, N. Y., assigner to Radio Corporation ofAmerica, a corporation of Delaware Application April 13, 1938, SerialNo. 201,736

4 Claims.

My present invention relates to automatic frequency control (A. F. C.)circuits, and more particularly to an improved and highly smpliiied A.F. C. circuit which possesses a wide frequency 5 control range atdiferent settings of the tuning device.

In application Serial No. 151,794, led July 3, 1937 by G. Mountjoy andD. E. Foster, granted Aug. 30, 1938 as U. S. P. 2,128,661, there hasbeen disclosed an A. F. C. circuit which utilizes the I. F. ampliner asa direct current amplifier for the A. F. C. bias. The amplified bias isemployed to vary the potential of the oscillator grid electrode of apentagrid converter thereby to adjust the frequency of the oscillatortank circuit. Further experimentation and investigation has revealedthat the frequency shift action on the tank circuit can be increasedwithout increasing the number of tubes, and without interfering withnormal receiver construction.

It may be stated that it is one of the main Objects of my presentinvention to produce a greater percentage shift in tuning of theoscillator tank .circuit without increasing the elements of theoscillator network.

Another important object of this invention is to permit (A. V. C.)automatic volume control action of the converter and I. F. amplifiertubes; and the A. F. C. bias being amplified by a special direct currentamplifier device provided in the same tube envelope as a discriminatordiode.

Another object of my invention is to provide in an A. F. C. circuit amultiple duty tube having a diode acting as a second detector and A. V.C. 35 bias source; an amplifier section to amplify the audio Output ofthe detector diode; and a second diode which cooperates with anindependent tube to provide a frequency discriminator network.

Still another object of my invention is to provide in a multiple pushbutton-trimmer condenser receiver an A. F. C. circuit capable ofeffecting satisfactory control over the oscillator tank circuitfrequency; and the control circuit embodying no special tubes.

The novel features which I believe to be characteristic of my inventionare set forth in particularity in the appended claims; the inventionitself, however, asr to both its organization and method of operationwill best be understood by 'reference to the following description takenin connection with the drawing in which I have indicateddiagrammatically a circuit organization whereby my invention may becarried into effect.

Referringnow to the drawing, which shows a superheterodyne receiversystem of compact (Cl. Z50-20) and eiiicient design, there is shown asignal collector A feeding signals, which may be in the broadcast rangeof 500 to 1500 kc., to the signal grid III of the converter tube 6. Thelatter may be of the 6A8 type. The signal collector can be a groundedantenna circuit; a tap to a radio frequency distribution line; a loopantenna; or it can be the usual pick-up device employed in connectionwith automobile receivers. This particular receiver is shown employingtrimmer condensers as the tuning instrumentality. Thus, numeral Idenotes the tuning coil of the signal input circuit, one end of coil Ibeing connected to signal grid I and the opposite end being connected:to the junction of coil 2 and condenser 3. The antenna A is connected toground through a condenser I and coil 2 which are in series resonancewith the I. F., they provide a trap for all undesired signals of the I.F. value. The coil 2` is connected to the high potential side ofcondenser I' through a path comprising coil 3 and condenser 4. The coil2 is shunted by a condenser II which resonates it to a frequency equalto the highest image frequency of the signal range. For example, with anI. F. value of 460 kc. the network 2 4 is tuned to 2420 kc. Thecondenser 4 is a direct current blocking condenser, while coil 3functions to choke image frequencies with respect to harmonics of thelocal oscillator frequency.

The condenser 3 couples the antenna network to coil I. The numeral 5designates, in a purely schematic manner, a gang of trimmer condensersof different values. Those skilled in the art are fully aware of themanner of selectively connecting a desired ltrimmer condenser acrosscoil I to tune the circuit to a desired signal carrier frequency. Thenumeral 5 denotes such selective means. For example, if desired, eachtrimmer condenser may have its associated push button for selectiveconnection of the trimmer across the coil I. Each trimmer, whenselected, is connected between the grid end of coil I and ground.

The tube 6 comprises the cathode 1, and the latter is connected toground by a resistor |23 shunted by the 0.2 mf. bypass condenser 23.Between the plate I I and cathode 'I there are disposedthe oscillatorgrid electrode 8, the oscillator anode electrode 9 and the signal inputgrid I0. The screen grids, which surround the signal grid I0, areconnected by lead I I to the Voltage reduction resistor IUI); the lowpotential end of the latter is connected to the cathodes of tubes 6 and|02 by lead I0 I. The high potential endof resistor |00 is connected toa proper positive potential point on the direct current voltage supplybleeder of the system.

The oscillator tank circuit comprises the coil IE and a selected one ofthe gang of trimmer condensers IS. Coil I5 is connected at one endthereof to electrode 9; the other end is connected to ground through a0.05 mf. condenser It. Each of the trimmer condensers I5 may beconnected in shunt across coil I5; it is to be understood that thenumerals IG-I designate a gang of selectively-operable trimmers similarto the gang 5 5. The dotted line P denotes a common actuating mechanismfor concurrently selecting trimmer condensers in the signal and tankcircuits which will producethe desired I. F. of 460 kc. Positivepotential is applied to electrode 9 from` a point (250 volts) on thesupply bleeder, the resistor I8 of about 20,000 ohms connecting coil I5to such positive point.

'I'he oscillator grid 8 is connected to ground' through a path includingcondenser 20, of 10 mmf., and coil I9. The latter is magneticallycoupled, as at M3, to tank coil I5. The signals and local oscillationsproduce I. F. energy in the resonant output circuit I3-|2; the latter istuned to the operating I. F. value. Those skilled in the art are fullyaware of the action o-f conversion which takes place in tube 6. Briefly,the electron stream of the tube is modulated by both the signal andoscillation frequencies; electronic coupling is employed to produce theI. F. energy. The coil I2 is coupled to coil 3| in the inputcircuit ofI. F. amplifier tube |02 which may be of the 6K7 type. The input circuitis resonated tothe operating I. F. value by condenser 3|'.

The plate of amplifier |02 is connected to a source of positivepotential through coil 36; `the condenser 37 tunes the latter to theoperating I. F. Coil 36 ismagnetically coupled, as at M1, to the coil 38of the discriminator input circuit. Coil 38. has its midpoint connectedto the plate side of coil 35. by condenser 39. Condenser 38 resonatescoil 38 to the I. F. value. The discriminator diodes are provided in twotubes 40 and 50. Tube 40 is a double triode of the GFS type; tube may bea double diode-triode of the 6Q7 type.

The grid and plate of one triode in tube 40 are strapped togetherproviding an anode 4I which cooperates with cathode 4I to provide adiode. Cathode 4I' is connected to ground through vresistors |00 andIBI', each resistor having a magnitude of about 1 megohm. The junctionof resistors |00 and IilI is connected to the midpoint on coil Baby lead155. The anode 4I is connected to one side of input circuit 38-38; theopposite side of theinput circuit is connected to the anode i2 of tube50. The cathode d2 of the latter is grounded; hence, the resistorsIDW-ISI are connected in series between the cathodes 4I and ft2. DiodeI2- 42' provides the second rectifier of the discriminator network.

The remaining triode of tube 4I) comprises the cathode 1I), control gridII and plate 12. The cathode "I0 is connected to ground throughresister` 13, the latter being bypassed by the 0.1 mf. condenser 1li.The grid II is connected to the cathode end of resistor |00', a 0.1 mf.condenser II bypassing the connection to ground. Piate 'I2 is connectedto a desired positive potential point on the voltage supply bleeder. Theresistor 'I3 is given a magnitude such that the cathode end thereof isapproximately 9 volts above ground. The triode 'III--TI-'IZ providesadirect current voltage amplifier; the A. F. C.

. bias developed at point 99` of resistor |00 is amplied by theamplifier 'III-'II-'I2, and the variable potential across resistor I3 isapplied to grid 8 of the converter tube 6 by lead l5. The latterincludes a filter resistor 16, and comprises the A. F. C. connection tothe oscillator section of the converter network.

The audio and A. V. C. voltages are developed by the diode 5|-2' of tube5I). The anode 5| is connected to the cathode 42 through a pathincluding link coupling coil 53, lead 53 and resistor 54. The coil 53 isof many turns, and is magnetically coupled, as at M2, to coil 33 at themidpoint thereof. The anode end of resistor 5ft is connected to cathode02 by the I. F. bypass condenser 50. The audio voltage across resistor5d is impressed on the grid 80 of audio amplifier section 42 0-2I. Thegrid 80 is adjustably tapped, as at 82,'to the load resistor 50; theusual audio potentiometer arrangement is employed in this connection.The amplified audio voltage may be transmitted toone, or more, audioamplifiers .and a nal reproducer.

The A. V. C. voltage is derived by connecting lead 90, including thepulsation voltage filter 9|, to the anode end of resistor 51|.- Lead 90is connected to the low potential end of coil 3|; a secondplead 92`connects lead 90 through resistor |03 to thejunction of condenser 4' andcoil 3. Hence, A. V. C. bias is applied to the signal grids of theconverter tube 6 and I. F. amplifier |02. Resistor 23 connects lead 90to B+, and provides +9 volts between lead 90 andground. Both signalgrids have a minimum negative bias established by potentialdifferencebetween the cathodes and grids thereof. The minimum negativebias will be -3 volts; as the carrier amplitude increases,` the negativebias will increase and reduce thegain of each of tubes 6 and |02 so asto maintain the carrier amplitude substantially uniform at the detectorinput circuit regardless of at either end of coil 38,*with respect toits midpoint, are 180 degrees out of phase. Hence, if themidpoint isconnected to the high potential sidev of coil 36, one potential isrealized which maximizes above the resonant frequency of the I. F.value, and a second potential is realized which maximizes below thisvalue. If these two potentials are applied. to a pair of rectiers, suchas the diodes I-M and l2-42', and the resulting direct current voltagesare added in opposition, the sum will be equal to zero. In the type ofdiscriminator network shown in the drawing, the primary and secondarycoils 36 and 38 are so connected that two vector sum potentials of theprimary and secondary voltages may be realized.y When the I. F. energydeparts in frequency Value from the assigned operating I. F., then thereis developed across resistors |00 and IllI a direct current voltage.Further description of this network is not believed necessary, since theoperation of the discriminator has been fully disclosed by S. W. Seeleyin application Serial No. 45,413,1iled Oct. 17, 1935 granted June 21,1938 as U. S. P. 2,131,103. The polarity of point 99 depends on thesenseof frequency shift of the I. F. energy, and the potential magnitude` ofpoint 99 depends on the amount of frequency departure. Since grid 'II isconnected to point 99, thecathodeend of resistor 'i3 varies in potentialaccordingto the variation of point 99. The cathode-end'of resistor'I3-will vary in a positive or negative sense with respect to thepredetermined |9 volts value. Hence, the grid 8 of the oscillator willvary similarly in potential with respect to its normal -3 volts bias.The +9 volts value of the cathode end of resistor 13 is chosen tocorrespond to correct tuning of the tank circuit l5-I6 to an oscillationfrequency which produces the assigned I. F. It will thus be seen that adeparture of the I. F. energy from the assigned I. F. value (in thiscase 460 kc.) results in a variation of the oscillator grid bias.Furthermore, the variation of the latter is in a sense such that theoscillator tank frequency is adjusted to compensate for the departure.

The separate diode rectifier .ral-42', arranged in tube 58, is employedso as to secure adequate selectivity even though but one stage of I. F.amplication is employed. The diode 5I--42' of tube 50 has its inputcircuit coil 53 coupled to the coil 33, and it will be seen that theinput circuit of the second detector diode is really a tertiary circuitcoupled to the secondary circuit including coil 38. The coupling betweencoils 3S and 53 is arranged so that coil 53 is coupled only to coil 38,and not to coil 36. This is done, as schematically shown in the drawing,by coupling turns 53 close to coil 38. In this way, with the use of buta single I. F. amplifier, the selectivity preceding the audiodemodulator is satisfactory.

The following explanation of the manner in which the changes in bias ofoscillator grid 8 are converted to frequency changes of the oscillatortank circuit follows: The oscillator produces an oscillatory voltageacross the tank circuit I5-I6 which is transferred to coil i9 by mutualinductance M3. In an oscillator the grid draws current so that there isa finite grid impedance, predominantly resistive, if the grid-cathodestatic capacity can be neglected. For the purposes of explaining theaction this capacity will be neglected, and its effect will be shownlater.- The voltage across the grid coil I9 causes a current to flowthrough capacity 20 and the input gridv resistance of grid 8 of tube 6.Capacity 20 and the grid resistance rg of the tube rotate the phase ofthe current relative to the voltage in that circuit. The current flowingthrough rg, therefore, has an in-phase and a quadrature component. Thein-phase component serves to maintain oscillation, and the quadraturecomponent serves to Vary oscillation frequency. These two components ofcurrent flowing through rg produce the grid voltage at oscillatorfrequency which, by virtue of the mutual conductance of the tube 8,produces an alternating current in the plate circuit thereof. Thedirection of the mutual inductance M3 between coils l5 and I8 necessaryto maintain oscillation is such that this alternating plate currentproduces an effect equivalent to a negative resistance and a negativecapacity in shunt to the tank circuit. i rThe negative resistancecomponent is responsible for the oscillation, and the negative capacitycomponent varies the frequency to some value other than that due to theconstants of the tank circuit alone. Now, when direct current voltage isapplied to grid 8 of tube 6, the effect on tube 5 is to vary the mutualconductance and the grid resistance rg. If the direct current bias ongrid 8 is made more positive, the mutual conductance increases but thegrid resistance decreases. The effect of these two factors beingopposite, tends to maintain the oscillatory voltage substantiallyconstant unless the bias on grid 8 is made so far positive that theoscillator ceases to function. If the direct current bias on grid 8 ismade negative, the negative capacity due to the quadrature componentchanges. In this case rg increases and the negative capacity in shunt tothe tank circuit decreases, and the oscillatory frequency decreases.

An increase in negative capacity in shunt to a tuned circuit has thesame effect on frequency as a decrease in a positive inductance in shuntto the tuned circuit. However, a negative capacity has an impedancewhich decreases with increasing frequency, whereas a positive inductancehas an impedance which increases with increase in frequency. Thefrequency shift produced by a given change in negative capacity is thusgreater at the high frequency end of the tuning spectrum where the tankcircuit capacity is low, whereas the effect of a given change inpositive inductance is a substantially constant percentage of thefrequency over the tuning spectrum since the tank circuit inductance isa constant. It has been shown that making the potential of grid 8 morepositive results in an increase in oscillatory frequency. If the directcurrent potential of grid 8 is made more negative the frequencydecreases. It has been shown, also, that the effect of capacity 28 is toproduce a negative capacity in shunt to the tuned circuit. If capacity28 is replaced by an inductance the effect would be as if a negativeinductance were shunted across the tuned circuit, in which case theamount of shift would be more constant over the tuning spectrum.

The tank coil and each associated trimmer condenser are made high inreactance (that is, by using a large coil and small condenser) so thatthe shunting effect of the anode quadrature current operates across ahigh impedance and produces a greater percentage shift in tuning of theoscillator tank circuit. The mutual Mz is sufficiently great to producevigorous oscillation and increased quadrature current. In general themutual M3 has this effect: increase in M3 produces a greater increase inshift at the low frequencies than the high. Hence, Ms may be so chosenas to obtain substantially constant shift at all frequencies of theband. As M3 is increased by increasing the value of coil I8, the latterwill tend to approach resonance at the high frequency end of the bandthrough the action of the capacities across it, and shift at the highfrequencies will be materially reduced. M3 may be arranged to provideconstant shift; or accentuated shift (with M3 very large) at the lowfrequency end of the band; or accentuated shift (with M3 small) at thehigh frequency end of the band.

It will be noted that the discriminator network is fundamentally similarto that described in the aforesaid Mountjoy and Floster application;however, it differs from the prior discrlminator circuit in that withthe discriminator tube and the demodulator tube there is secured anadded function. This added function is the direct current amplificationof the A. F. C. bias derived from the double diode network. Accordingly,it will be observed that A. V. C. action may be applied to the I. F.amplifier preceding the discriminator and demodulator. The directcurrent amplifier has a limiting action. Initially, with the correctstation tuning, the amplifier provides a voltage across resistor 13equal to the normal plate current flow under self-biasing conditionsmultiplied by the value of resistor 13.

sof

' resistor i3 increases.

plifier grid 'H draws grid current and shunts out the two resistorsIdil' and lill' which are in series with the grid, the latter elementsproviding the internal impedance of the. discriminator output. Hence,the positive increase of the drop across resistor lis limited to a ValueVery near that of resistor 13 multiplied by the zero bias current of thetriode -1|-'I2.'

To explain the receiver operation, and specifically the functioning ofthe A. F. C. circuit, whenan input signal is applied to antenna A, oflower frequency than is necessary to produce by beating with theoscillator frequency an` I. F. of predetermined value, an I. F. signalis produced which is higher than the predetermined frequency. Signals ofthis higher carrier frequency are passed through the discriminatorcircuit and a discriminator voltage is. developed acrossresistors Hilland |01; the voltage is negative as applied to the control grid .'H oftube Mi.' A decrease in cathode current in tube 46 results in a decreasein voltage across resistor 'i3' thus changing the bias on grid 8 of tube6 and producing a more negative bias on grid 3. This more negative biasshifts the frequency of oscillation as previously explained, and causesa lower frequency to be produced. The lower oscillator frequency beatingwith the input signal produces a decrease in the I. F. signal frequency,and tendsv to correct for the assumed condition of I. F. signals higherthan the desired I. F. With an input signal to the antenna that ishigher in frequency than that necessary to produce the correct I. F.value by beating with the `oscillator frequency, the reversal of theabove ybiasing action takes place and the I. F. value is corrected. Thecontrol characteristics of the oscillator control circuit wereexperimentally measured on a receiver from which the schematic diagramwas derived.

While I have indicated and described a system for carrying my inventioninto effect, it will be apparent to one'skilled in the art that myinvention is by no means limited to the particular organization shownand described, but that many modifications may be made without departingfrom the scope of my invention, as set forth in thel appended claims.

What'l.' claim is:

l. In a superheterodyne receiver of the type employing a localoscillator circuit, said` circuit comprising a tube having at least acathode, a control grid and an anode electrode, aftunable oscillationtank circuit connected to said anode and cathode and being reactivelycoupled to said grid, means-for varying the direct current potential-ofthe oscillator grid electrode, a single condenser element connectedbetween the grid and cathode, said element being so chosen inmagnitudeas to reflect across said tank circuit a capacitative effectwhich is adapted to vary the frequency of the oscillator tank circuit inresponsev to variations in the direct current potential of saidgrid, anintermediate frequency amplifier having an input circuit .andan outputcircuit, a discriminator network havingan input circuit coupled to saidamplifier output circuit, saiddiscriminator having a direct currentvoltage output circuit, adirect current voltage amplifier having inputand output circuits, a connection between-the discriminator outputcircuit and the input circuit of the direct current voltage amplifier,and a direct current voltage connection between the output circuit ofsaid direct current voltage amplifier and said oscillatorv gridproviding said varying means.

2. In'combination with an electron discharge tube of the type includingat least a cathode, an anode electrode and a control electrode, aresonant tank `circuit connected between the cathode and one of said twoelectrodes of the tube, meanstfor reactively coupling the secondelectrode to said tank circuit, solely a condenser external of said tubeconnected between said cathode and said second of the tube electrodes,said condenser having a magnitude such that it rotates the phase of thecurrent flowing in the circuit of said second electrode with respect tothe alternating voltage applied thereto thereby varying the effectivereactance of said tank circuit, and means for Varying the direct currentpotential of said second electrode thereby to control the effectivenessof said condenser.

3. In an oscillator network of the type including a tube provided withat least a cathode electrode, an anode electrode and a controlelectrode, a resonant tankcircuit connected between the cathode and saidanode electrode, purely magneti'c means coupling said anode and controlelectrodes in such a manner as to provide an oscillatory current flowthrough said tank circuit, a single reactive element external of saidtube connected between said cathode and control electrode, said reactiveelement being chosen to rotate the phaseof the current flowing in thecircuit of said control electrode with respect to the alternatingvoltage applied thereto thereby to vary the effective reactance of saidtank circuit, and means for varying the direct current potential of saidcontrol electrode thereby to control the effectiveness of said reactiveelement.

ll.l In combination, a tube including at least a cathode, an anode and acontrol electrode, the anode including in circuit therewith a coil ofrelatively high inductance value, a plurality of independent condensersof different values arranged for selective shunting across the coil,means providing a magnetic coupling between said control electrode andsaid coil whereby oscillatory currents are caused to ow through said`coil, the frequency of said currents being dependent upon the condenserin shunt with said coil, a condenser in series between the controlelectrode and cathode for rotating the phase of current flowing in itscircuit with respect to the alternating voltage applied thereto wherebya capacitativey effect is produced across said coil which issupplemental to said shunt condenser, means for varying the directcurrent potential of the control electrode thereby to control the valueof the said effect over a desired magnitude range, and said magneticcoupling being chosen to provide a predetermined rate of variation oversaid range.

GARRARD MOUNTJ OY.

